This invention relates to control circuitry for DC motors, and is more particularly directed to a control circuit for a capstan motor of a tape recording and/or reproducing device, especially a control circuit of a type adapted to control the motor so that the latter drives the tape intermittently in either a forward or a reverse direction.
In video recorders, particularly video tape devices in which fields of video information are recorded on successive slant tracks on the tape, it is often useful to drive the tape intermittently so that different slant tracks can be repeatedly scanned, such as in slow-motion or still-frame reproduction. This is particularly useful, for example, when editing a video program recorded on tape.
On video tape, control signals are typically recorded in a longitudinal control track along an edge of the tape to indicate the position of respective associated slant tracks. That is, the control signals are situated in predetermined locations on the tape relative to the location of the slant tracks. Accordingly, the control signals, which are detected by a stationary control head as the tape is drawn past it, can be used to control stopping of the capstan during intermittent operation and to stop the tape at the location of a desired slant track. This ensures that the trace of a rotary head, carried on a rotating head wheel or drum for picking up the video signal, coincides accurately with the desired slant track. It is preferred that this be true for intermittent operation in both the forward and the reverse direction of transport of the tape.
A conventional capstan motor control circuit generates a drive pulse to effect intermittent motion of the capstan motor. This drive pulse commences (rises to high level) at the time a start signal is received and then ceases (drops to low level) at a later time estimated to correspond with the advancement of the tape by one slant track. A braking pulse occurs when the control signal for a desired track is detected, and is used to halt rotation of the capstan motor. This pulse has a width calculated to be sufficient to stop the motor, but short enough not to cause reverse rotation thereof.
This conventional control circuit has not been capable of accurately halting the tape at the positions of the recorded slant video tracks. For example, if there is drop out in the reproduced control signal, the tape cannot be stopped until the next control signal is picked up. This can result in mistracking. Further, because the brake pulse cannot be generated until the control signal has been picked up, if the desired stopping position is near the position of the recorded control signal on the tape, the tape drive mechanism will advance the tape beyond the desired position before the tape can be stopped, which also can result in mistracking.